1. Field of the Invention
This invention relates to mass spectrometry (MS), particularly to quadrupole ion trap mass spectrometry, an analytical method wherein a sample is analyzed via measuring masses of ions and their fragments.
2. Description of the Related Art
A quadrupole ion trap mass spectrometer (ITMS) is a versatile analytical device capable of MS and multiple MS (MS″) measurements in a small package. Because of its small size it is frequently used in portable (field deployable) mass spectrometry applications. The ion trap cell of the ITMS instruments may have different geometry leading to different ITMS types. One common ion trap is a three-dimensional (3D) trap using one ring and two cap electrodes for generating the trapping quadrupole field inside the ion trap cell. Another common ion trap is a linear ion trap in which the ions are trapped within an ion trap cell having four linear rod electrodes and two trap electrodes placed at the ends of the four-rod structure. Examples of other ITMS or quadrupole ion trap-based types are toroid, cylindrical and rectilinear ion trap mass analyzers or a hybrid ion trap/time-of-flight mass analyzer.
In a quadrupole ion trap mass spectrometer the ions to be analyzed are first trapped inside an ion trap cell and then mass-analyzed by sequentially ejecting those (typically using resonance excitation) toward an ion detector via scanning RF/DC voltages applied to the trap electrodes for generating the trapping field. An important step of the ITMS operation is the cooling down of the trapped ions toward the ion trap cell center (where the trapping potential is minimal) before the ion ejection step. This is done in collisions of ions with buffer gas molecules which are present in the ion trap cell at typical pressure of 10−4-10−3 Torr. ITMS is the only MS technique that can operate at such high pressures which is another reason why such analyzers are frequently used in portable MS systems as low vacuum reduces system pumping requirements. The buffer gas molecules have another function in tandem MS (MSn) experiments where these molecules serve as a collisional partner to break down (to fragment) ions into product (fragment) ions. The requirements for selection of buffer molecules required in cooling and fragmentation steps are different. In the cooling step the buffer molecules should be as light as possible because heavy molecules can interfere with the following ejection steps by breaking down the coherent motion of the resonantly excited ions resulting in low mass resolution. In the fragmentation step, the highest fragmentation efficiency is achieved when the mass of the colliding partner/buffer molecule is as high as possible (V. M. Doroshenko, R. J. Cotter, Pulsed Gas Introduction for Increasing Peptide CID Efficiency in a MALDI/Quadrupole Ion Trap Mass Spectrometer, Anal. Chem., 1996, v. 68, 463-472).
Traditionally helium (MW=4 Da) is mainly used as a buffer gas in ITMS instruments. This light gas provides a good mass resolution in a broad mass range (typically up to 2,000 Da) and high fragmentation efficiency for peptide ions (typically analyzed in proteomics, a widespread ITMS application). The use of helium in portable MS devices has been problematic because of the heavy weight of gas cylinders used for storage of compressed gases and restrictions imposed on transportation of the compressed gases, especially by air shipment. For this reasons, substantial efforts have been taken to replace helium for air readily available from the environment (R. M. Danell, A. S. Danell, G. L. Glish, R. W. Vachet, “The use of static pressures of heavy gases within a quadrupole ion trap,” J. Am. Soc. Mass. Spectrom., 2003, v. 10, 1099-1109). In general, some ITMS instruments can work with air as buffer gas but this severely reduces their mass resolution especially at high mass range.
The goal of the present invention (not realized before this invention) is to get rid of a heavy and large compressed gas cylinder typically providing ITMS with a helium buffer gas while maintaining ITMS performance at the level similar to that with helium buffer gas. The additional goal (not realized before this invention) is to provide easy transportability of ITMS by airplane that is important for a portable MS instrument.
The content of all sources cited herein is hereby incorporated by reference.